Batch thermal processing continues to be used for several stages of fabrication of silicon integrated circuits. One low temperature thermal process deposits a layer of silicon nitride by chemical vapor deposition, typically using chlorosilane and ammonia as the precursor gases at temperatures in the range of about 700° C. Other, high-temperature processes include oxidation, annealing, silicidation, and other processes typically using higher temperatures, for example above 1000° C. or even 1350° C.
For large-scale commercial production, vertical furnaces and vertically arranged wafer towers supporting a large number of wafers in the furnace are typically used, often in a configuration illustrated in the schematic cross-sectional view of FIG. 1. A furnace 10 includes a thermally insulating heater canister 12 supporting a resistive heating coil 14 powered by an unillustrated electrical power supply. A bell jar 16, typically composed of quartz, includes a domed roof 18 and fits within the heating coil 14. An open-ended liner 20 fits within the bell jar 16. A support tower 22 sits on a pedestal 24 and during processing the pedestal 24 and the support tower 22 are generally surrounded by the liner 20. The tower 22 includes three or four vertically extending legs 26 fixed at the ends to a tower top plate 28 and a tower bottom plate 30. The legs 26 include vertically arranged slots 32 for holding multiple vertically arranged and horizontally disposed wafers 34 to be thermally processed in batch mode. If used, one or more gas injectors 36 principally disposed between the liner 20 and the tower 22 have multiple gas outlets 38 at different heights for injecting processing gas within the liner 18. An unillustrated vacuum pump removes the processing gas through the bottom of the bell jar 16. Conventionally, the top of the liner 20 is left open so that, in processing, the processing gas generally flows upwardly within the liner 20 and out the top of the liner 20 before descending downwardly around the outside of the liner 20 to the vacuum pump. The heater canister 12, bell jar 16, and liner 20 may be raised vertically to allow wafers to be transferred to and from the tower 22, although in some configurations these elements remain stationary while an elevator raises and lowers the pedestal 24 and loaded tower 22 into and out of the bottom of the furnace 10.
The bell jar 16, which is closed on its upper end by the dome 18, produces a generally uniformly hot temperature in the vertically middle and upper portions of the furnace 10. This is referred to as the hot zone in which the temperature is controlled for the optimized thermal process. However, the open bottom end of the bell jar 16 and the mechanical support of the pedestal 22 causes the lower end of the furnace 10 to have a lower temperature, often low enough that the thermal process such as chemical vapor deposition is not effective. The hot zone may exclude some of the lower slots of the tower 22.
Conventionally in low-temperature applications, the tower, liner, dome, and injectors have been composed of quartz, typically fused silica. However, quartz towers, liners, and injectors are being supplanted by silicon towers, liners, and injectors. Towers of silicon carbide are also available. Silicon towers having somewhat different configurations for various applications and silicon injectors are commercially available from Integrated Materials, Inc. of Sunnyvale, Calif. and are disclosed respectively by Boyle et al. in U.S. Pat. No. 6,450,346 and by Zehavi et al. in U.S. patent application Ser. No. 11/177,808, filed Jul. 8, 2005 and published as U.S. Patent Application Publication 2006/0185589, now abandoned. Silicon liners based on silicon staves bonded together into a generally tubular (actually polygonal) shape are disclosed by Boyle et al. in U.S. patent application Ser. No. 10/642,013, filed Sep. 26, 2001, published as U.S. Patent Application Publication 2004/0129203, and issued as U.S. Pat. No. 7,137,546, and incorporated herein by reference. A preferred liner design disclosed by Reese et al. in U.S. patent application Ser. No. 11/536,352, filed Sep. 29, 2006 and now published as U.S. Patent Application Publication 2007/0169701, includes generally rectangularly shaped staves having interlocking structures on their shared edges. All these patent references are incorporated herein by reference. Silicon is available in very high purity in the form of virgin polysilicon (electronic grade silicon) and thus contains very low levels of impurities. However, a silicon member is defined as comprising at least 95 at % and preferably at least 99 at % of elemental silicon.
The use of a silicon tower, a silicon liner, and silicon injectors allows the hot zone to include only silicon parts, which substantially reduces contamination and particles. However, it is now believed that the dome 18 of the quartz bell jar 16 generates a significant number of particles in the 0.2 to 1 μm and larger range, which may fall into the open end of the liner 20 and thence onto the tower 22 and its supported wafers regardless of the composition of the liner 20 and tower 22. Any die on which a particle falls is likely to be inoperable or at least undependable. That is, particles severely affect yield. However, sealing the upper end of the liner 20 would prevent process gas from the injectors 34 from flowing in a regular pattern past the tower 22 and its supported wafers 34.